1. Field of the Invention
The present invention relates to a liquid-crystal display panel having a touch-sensor function and a chopper-type comparator which can be properly installed on such a liquid-crystal display panel.
2. Description of the Related Art
In recent years, there have been proposed a variety of display panels provided with a sensor function embedded in liquid display panels as a function which allows inputs to be entered to the display panels by the user through operations to touch the panels.
However, most of the proposed display panels adopt a method described as follows. The output of a touch sensor for implementing the sensor function is fetched to the outside of the display panel as an analog signal. The analog signal is then subjected to an A/D (analog/digital) conversion process which is carried out by an IC external to the display panel in order to convert the analog signal into a digital signal. Subsequently, the digital data is subjected to various kinds of data processing.
In this case, typically, a touch sensor is employed in each liquid-crystal pixel circuit included in the display panel. Thus, in order to detect information generated by each of the sensors, as many wires as the sensors need to be provided between the display panel and the external IC. As a result, the proposed display panels raise a problem that the frame of the display panel increases and the external IC becomes larger in size.
To put it more concretely, there has been proposed a method described in U.S. Published Application No. 2006/0262100 as a method of making use of an external IC for detecting the output of a touch sensor, which is employed in every liquid-crystal pixel circuit included in the display panel, through a signal line originally used for giving a display data signal to the pixel circuit.
FIG. 16 is an explanatory circuit diagram showing a typical configuration for implementing the method for detecting the output of a touch sensor which is also referred to as a pressure sensor in the following description. As shown in the circuit diagram of FIG. 16, the display panel is a pixel array 100 employing pixel circuits 101 which are laid out to form a two-dimensional matrix. The pixel array 100 also includes a number of data signal lines LS which are each oriented in the direction of the columns of the two-dimensional matrix. In addition, the pixel array 100 also includes a number of gate lines GL which are each oriented in the direction of the rows of the two-dimensional matrix. Each of the pixel circuits 101 is placed at the intersection of one of the data signal lines LS and one of the gate lines GL.
Each of the pixel circuits 101 has a liquid-crystal cell LC containing a liquid crystal provided between electrodes which face each other. To put it concretely, the liquid crystal is provided between a pixel electrode and a COM (common) electrode which is facing the pixel electrode.
In addition, the pixel circuit 101 also employs a pixel transistor Tr. The gate node of the pixel transistor Tr is connected to the gate line GL whereas the source node of the pixel transistor Tr is connected to the data signal line LS. The drain node of the pixel transistor Tr is connected to the pixel electrode of the liquid-crystal cell LC.
On top of that, the pixel circuit 101 also employs a pressure sensor S. The pressure sensor S has a structure for electrically connecting the pixel and COM electrodes to each other when an external pressure is applied to the pressure sensor S by a finger of the user.
Each of the data signal lines LS on the pixel array 100 is connected to an external IC 102. In the external IC 102, each of the data signal lines LS is connected to a circuit which includes an operational amplifier 121, an A/D converter 122, a switch 123 and a capacitor 124 as shown in the circuit diagram of FIG. 16.
The switch 123 is operated to change an operation to assert display data on the data signal line LS to an operation to read out information generated by the pressure sensor S from the data signal line LS and vice versa. To put it in detail, in a write operation to assert display data on the data signal line LS, the switch 123 is put in a turned-on state in order to assert the signal value of the display data on the data signal line LS. The signal value has been supplied to the switch 123 from a write circuit by way of the operational amplifier 121. The signal value asserted on the data signal line LS is written into the liquid-crystal cell LC of the pixel circuit 101 by way of the pixel transistor Tr which has been put in a turned-on state by the gate line GL. It is to be noted that the write circuit itself is not shown in the circuit diagram of FIG. 16.
In a read operation to read out information generated by the pressure sensor S from the data signal line LS, on the other hand, the switch 123 is put in a turned-off state in order to electrically disconnect the output of the operational amplifier 121 from the data signal line LS. In this state, information output by the pressure sensor S is amplified by the operational amplifier 121 and, then, an analog signal generated by the operational amplifier 121 is converted by the A/D converter 122 into a digital signal.
In contrast with the configuration described above, a technique for reducing the number of lines connecting the liquid-crystal display panel 100 to the external IC 102 is adopted by, for example, a small-size display panel for mobile apparatus or the like. FIG. 17 is a diagram showing a typical configuration of a display panel in which the number of lines connecting the liquid-crystal display panel 100 to the external IC 102 is reduced in order to decrease the size of the frame of the display panel 100 and the chip size of the external IC 102. In order to reduce the number of lines connecting the liquid-crystal display panel 100 to the external IC 102, the display panel adopts the so-called selector method in carrying out signal write operations. Such a display panel has been becoming popular.
In the typical configuration shown in the diagram of FIG. 17, a switch 103 associated with the B (blue) color, a switch 104 associated with the G (green) color and a switch 105 associated with the R (red) color are provided for every three data signal lines LS respectively. The three data signal lines LS are connected by the switches 103, 104 and 105 to a single data signal line LSrgb which is connected to the external IC 102.
To put it more concretely, the switch 103 associated with the B color is provided for a data signal line LS connected to a B (blue) pixel circuit 101. By the same token, the switch 104 associated with the G color is provided for a data signal line LS connected to a G (green) pixel circuit 101. In the same way, the switch 105 associated with the R color is provided for a data signal line LS connected to an R (red) pixel circuit 101.
In this case, for any B, G and R pixel circuits 101 connected to a gate line GL serving as a horizontal row of the two-dimensional matrix and connected to data signal lines LS each serving as a vertical column of the two-dimensional matrix, the B, G and R switches 103, 104 and 105 are put in a turned-on state on a time division basis during one horizontal period in order to store B, G and R signal values from the data signal lines LS into the B, G and R pixel circuits 101 respectively.
In such a configuration, the number of single data signal lines LSrgb each connecting the liquid-crystal display panel 100 to the external IC 102 through the switches 103, 104 and 105 each serving as a selector is smaller than the number of data signal lines LS. That is to say, the number of lines connecting the liquid-crystal display panel 100 to the external IC 102 can be reduced. In particular, the larger the number of selectors cited above, the larger the number of eliminated lines connecting the liquid-crystal display panel 100 to the external IC 102 and, thus, the larger the effect of the reduced number of such lines. As a result, the larger the number of selectors cited above, the greater the obtained cost merit.
However, the larger the number of selectors cited above, the shorter the write time. The write time is defined as the time during which a driver circuit embedded in the external IC 102 is connected to a data signal line LS in the pixel array 100.
By the same token, the larger the number of selectors, the shorter the read time. The read time is defined as time that it takes to carry out the read operation described earlier by referring to the circuit diagram of FIG. 16. Thus, if an attempt is made to share a data signal line LS among pieces of information read out from touch detection sensors S as described earlier by referring to the circuit diagram of FIG. 16, the read time becomes extremely short and the proposed method becomes virtually impractical.